Gate turn-off devices are known in the art. Generally, a thyristor can be turned on with a small gating signal of one polarity, say positive. Since a thyristor is a regenerative element, it will stay conductive above a certain minimum holding current flowing between its main terminals, even if gate drive is removed. Such thyristor can be turned off by a negative gating signal. For low level conduction current through the thyristor, say only slightly above the minimum holding current, only a small negative gating signal is necessary to turn it off. However, when the thyristor is conducting heavily with a high amount of current flow therethrough, turn-off is difficult or impossible and requires a substantially larger negative gating signal.
Numerous schemes are known for accomplishing turn-off. One approach is to supply the requisite high level off signal by means of a short duration, high level pulse, for example by capacitor discharge.
Another approach is to attack the problem from a different angle and attempt to reduce the requisite magnitude of the off signal necessary to achieve turn-off. For example, rather than using a thyristor alone as the power switching element, it is instead used as a low level regeneration element to control a non-regenerative power element (i.e. continual gating or driving current is necessary to maintain conduction). The thyristor is connected to drive the power element into conduction such that the latter carries load current and the thyristor carries only a small on current which is above its minimum holding value and which is also above the minimum driving current necessary to maintain continued conduction of the power element. It is thus generally known to employ the combination of a low level regeneration element controlling a high current capacity non-regenerative element in order to achieve high turn-off gain by breaking the regenerative loop with a low level signal.
The integration of a vertical SCR and a vertical power transistor is known. The SCR and the transistor operate in parallel with the transistor carrying most of the load current. The base region of the transistor is common with one of the interior regions of the SCR, whereby when the SCR is conductive, there is supplied base drive to the transistor and the latter conducts. Since only a low level current flows through the SCR, it is easily turned off by a negative gating signal, which breaks the regenerative loop, thus terminating base drive for the transistor and the device is turned off.